It does do it, open your eyes.
The only time it should do it ever is a bug. And from what ive seen these very very VERY rare penetration issues are bugs the EiR team proboly cant fix. Proboly a issue with hardcoded engine stuff.
So basically this hetzer not penetrating a m10 has proboly only happened every million hetzer shots fired.
Which gets to be quite irrelevant to balance if its that rare.